Iron, the commonest and most useful metal, is always used commercially in the alloyed form, as its properties can be varied as to hardness, ductility, flexibility, tensile strength, chemical resistance, and other properties by the choice, amounts, and combination of alloying elements. Gray cast iron is distinguished by a relatively high amount of carbon, approximately 3%, which imparts to it the characteristic hardness, castability, wear resistance, and machinability displayed by no other metal.
Gray cast iron is unique in its high content of carbon, and in the form of a large portion of this carbon as a separate phase of graphite. The strength, wear resistance, bittleness or conversely toughness, and machinability are all controlled to a large and primary extent by the graphitic carbon content. Graphite in gray iron appears in several forms well-known to the foundry metallurgist, of which the so-called type A, a flake, is preferred, in a pearlitic iron matrix. If the carbon is present as iron carbide, or cementite, the metal will be what is known as white iron, hard, brittle, and unmachinable. If the carbon is present in the correct proportion as graphite in the pearlitic matrix, it will display the characteristic gray color and good machinability of gray iron.
(This treatment ignores the effects of the other alloying elements and heat treatment and will be limited to the effects of silicon and carbon upon the properties of gray cast iron, in order to simplify its complex subject matter.)
When gray iron is melted in a cupola over a bed of hot coke, it gains some carbon content from the coke, which may be varied by adjusting the coke-iron ratio, the air blast, by additives such as silicon, and by the slag chemistry.
When it is poured into the molds to produce parts, the utility of these parts is affected by the cooling rate, and the rate of precipitation from solution of the various forms of iron. An iron melt which hardens too quickly will have an excess of iron carbide and have the characteristics of white iron, hard, brittle, poorly machinable, and relatively strong.
If the iron has an excess of carbon as graphite with the metal predominantly in the form of primary ferrite from a too slow cooling rate, the metal will have low tensile strength and be too soft to be commercially useful.
The amount and shape, size, and distribution of graphite present in a gray cast iron are usually controlled by the addition of an inoculant to the metal in the cupola, the ladle, or the mold which furnishes seeds for formation of crystals of graphite. Inoculants commonly used are silicon in various forms, such as ferrosilicon or silicon carbide, and graphite itself. Other metals used include chromium, manganese, calcium, titanium, zirconium, aluminum, barium and strontium.
Some of the elements function as alloying elements as well, in particular molybdenum, chromium, and manganese. Aluminum and the alkaline earths are the most effective non-graphitic inoculants.
Silicon is the principal element used as an inoculant, controlling graphite formation, allowing the formation of the pearlitic iron matrix over a wider temperature range, and thus decreasing the chill depth of the cast metal.
The chill depth test is usually conducted by casting a graduated wedge-shaped test piece under specific conditions, and measuring the extent of the white iron from the tip of the wedge. Since the thinner portion cools faster, the tip will be of white iron or iron carbide, which will crystallize earliest, and is light colored, hard, brittle and unmachinable in normal operation. The extent of the chill depth controls principally the thickness of the casting which can be made from a particular melt, a melt with a low chill depth enabling a relatively thinner casting to be poured without the formation of white iron. A thick cross-sectioned casting is made with iron with a greater chill depth to avoid the formation of excess graphite and ferrite. The desired metal consists of graphite flakes in a matrix of pearlitic iron, which is stabilized over a widely varying cooling rate.
Past practice in this area has shown the use of silicon carbide as an added ingredient in the cupola charge or to the ladle by U.S. Pat. No. 2,020,171 and U.S. Pat. No. 2,119,521 to Brown.
The use of silicon carbide in briquette form is shown by U.S. Pat. No. 2,497,745 to Stohr; U.S. Pat. No. 2,527,829 to Leitten; U.S. Pat. No. 3,051,564 to Drenning; and U.S. Pat. No. 3,666,445 to Stone et al. U.S. Pat. No. 4,015,977 to Crawford claims briquettes of petroleum coke with refractory oxides or a derivative which will yield a metal oxide.
A clear explanation of the use of silicon carbide in gray iron melts is given by Moore, U.S. Pat. No. 3,764,298, showing desirable and undesirable grain structures and chill wedges with small additions of silicon carbide to the metal.
The commercial silicon carbide used in the practice of this invention is a by-product of the Acheson graphite process. When baked carbon electrodes are packed with resistor coke and then covered with a coke-silica mixture and electrically heated to transform the amorphous carbon to crystalline graphite, some of the silica reacts with carbon forming silicon carbide according to the following equation: EQU SiO.sub.2 +2C.fwdarw.SiC+CO.sub.2.
The commercial grade used in this invention contains approximately 50% to 60% graphite and 20-25% silicon carbide with the remainder a mixture of silicon dioxide and other metallic oxides.